Most living things exhibit striking seasonal changes during the course of the year. In many cases, the changes are known to be induced by seasonal changes in daylength, or photoperiod. Changes in the photoperiod generally affect animals in two ways. First, the constant daily expansion or contraction of the photoperiod can cause an animals daily activity or rest phase to expand or contract, so that it matches the photoperiod. Second, specific changes in the photoperiod can trigger new behaviors, such as hibernation, migration, or breeding, that represent adaptations to conditions that prevail during a particular season. These new behaviors are often accompanied by and depend on marked changes in the biochemistry, physiology and anatomy of the organism. According to a classic model of Pittendrigh and Daan, the photoperiod controls the organism through its effects on a complex circadian pacemaker that employs two separate oscillators to track the constantly changing times of dawn and dusk through the course of the year. This project is designed to investigate whether human biology is similarly subject to control by changes in the photoperiod. In the first phase of the project, we monitored daily patterns of sleep, hormones and body temperature in individuals who lived for one month on a schedule of light exposures that simulated a natural winter day, with ten hours of light and fourteen hours of darkness. For comparison, we made similar measurements in the same individuals while they lived for one week on a conventional schedule that simulated a natural summer day, with 16 hours of light and 8 hours of darkness. We found that the human circadian system is similar to that of other animals - that it is composed of two subsystems, one of which is synchronized with dawn and the other with dusk. This conclusion was based on the fact that the durations of nocturnal periods of various circadian rhythms (for example, the period from the onset to the offset of nocturnal melatonin and prolactin secretion) were longer after exposure to long nights than short nights. In a follow-up experiment, we investigated whether healthy individuals living in a modern, urban environment (metro Washington, DC area), in which they are routinely exposed to artificial light, are still able to detect seasonal changes in the duration of the night and to respond to these changes by modifying the duration of the nocturnal period of melatonin secretion. We found that patterns of nocturnal melatonin secretion in women responded to seasonal changes in the duration of the night, while those in men did not. This finding suggests that there may be gender differences in responsiveness to artificial light of mechanisms that track seasonal changes in the length of night.